Outer Gas Giants Rare?

by Paul Gilster on July 13, 2007

Centauri Dreams sometimes gets e-mail from readers asking how research results can be so contradictory. We’ve discussed gas giants around red dwarf stars, for example, noting theories that such planets are rare in this environment. And then we come up with stars like Gliese 876 and GJ 317, both red dwarfs, and both sporting not one but two gas giants as companions. But stand by, for in a moment we’ll look at new evidence that outer gas giants are indeed rare, and not just around M dwarfs.

What’s going on? The answer is that exoplanetary studies are a work in progress, and will continue to be as far into the future as I can see. We have identified over 200 exoplanets in a galaxy of several hundred billion stars. You bet we’re going to find anomalous situations that challenge every theory we have. And the idea is to put hard scientific work out there for review and critique, noting methodologies and explaining conclusions, thus letting other scientists have a go at the same data.

Those seeking conclusive answers this early in the game are going to find this frustrating, but that’s how science works, and it’s a measure of the complexity of what we’re studying that exoplanetary systems yield their secrets only slowly and over time. The new work for today is an example, a collaboration between US and European astronomers that surveyed 54 young, nearby stars thought to be candidates for Jupiter-class planets at distances beyond Jupiter’s own 5 AU from their star.

Radial velocity techniques are great for finding planets close to the stars they orbit, but much more problematic when dealing with outer planets. So the survey team worked with direct-imaging methods instead, and methane-sensitive imagers specifically designed for this operation. Their conclusion is a bit startling: The survey failed to find a single extrasolar planet in the outer parts of any of the nearby systems it studied. Says graduate student Eric Nielsen (Seward Observatory), “There is no ‘planet oasis’ between 20 and 100 AU. We achieved contrasts high enough to find these super Jupiters, but didn’t.”

One thing scientists will now look at as they probe this work is the imaging technique involved in the survey, based on an instrument called the Simultaneous Differential Imager (SDI) that has been used with both the ESO Very Large Telescope 8.2-meter instrument in Chile and the 6.5-meter telescope at the UA/Smithsonian MMT Observatory on Mount Hopkins, Arizona. The SDI camera splits the light from a single object into four images, which are then sent through methane-sensitive filters to a detector array. Ideally, the bright star disappears while the methane-laden companion comes into view, as shown below.

The method has had success in the past, discovering a brown dwarf around the star SCR 1845-6357, some 12.7 light years away. And Laird Close (University of Arizona), one of the developers of the SDI, finds it powerful enough to say this: “We certainly had the ability to detect outer super Jupiter planets at 10 AU, and farther out, around young sun-like stars.”

Image: Comparison of images taken with SDI on and off. A number of fake planets (at separations of 0.55″, 0.85″, and 1.15″ from the star) were added in to this data, which was then analyzed first using the SDI method and second, using standard adaptive optics techniques. The simulated planets, each seen as a pair of black-and-white dots 33 degrees apart in the SDI image, are easily detected yet are 10,000 times fainter than the central star in the standard adaptive optics analysis. Credit: Laird Close/University of Arizona.

But they didn’t. What would be the constraints on outer gas giant formation around these stars, and how does the survey result affect our current notions of planet formation? Work like this is interesting precisely because it targets filling in the gaps in our knowledge of outer exoplanetary systems, helping us ultimately to learn whether our own Solar System is somewhat average or a departure from the norm. And the answers to the questions it raises will be worked out over time and with the contribution of further surveys using a wide variety of technologies.

Surprises, then, are the nature of the game, and should be considered as opportunities to refine existing theories or suggest new ones. We’ll all watch this process at work as researchers study the two papers involved. They’re Biller et al., “An Imaging Survey for Extrasolar Planets around 45 Close, Young Stars with SDI at the VLT and MMT,” accepted by the Astrophysical Journal (abstract available) and Nielsen et al., “Constraints on Extrasolar Planet Populations from VLT NACO/SDI and MMT SDI and Direct Adaptive Optics Imaging Surveys: Giant Planets are Rare at Large Separations,” submitted to the Astrophysical Journal (abstract).

Zen, I suppose you could use the technique to confirm a gas giant found by another method, if the configuration is suitable. Except I believe those other methods tend to find gas giants much closer to the primary than this method can detect. That makes it complementary.

There is also value is having a technique that can generate a larger statistical sample of red dwarfs exoplanet non-existence, if only to exclude the likelihood of large separation gas giants. Not as exciting as finding more planets but useful in understanding these systems a little bit better.

I say all this without having read the abstract Paul linked to, so hopefully I’m not off base.

I believe I understand your point, but what would constitute proof? If a planet is found with this technique, that may not be proof since it could be an artifact due to unexpected problems with the method or the instrument. They appear to have at least shown that the characteristics of such a planet and primary is in principle detectable.

A lot of new methods for finding planets are about pulling a very weak signal out of a lot of noise. This seems no different.

Still, I don’t disagree with your concern. I suspect as with any method they will over time look to corroborate what they find with at least one other detection method, or perhaps refine the methods to reduce the likelihood of false positives and negatives.

All in al, I don’t find these research results very surprising, nor shocking (though interesting): it just means, that (super) giant planets are very rare or absent at very great distance from their mother stars (> 10 AU). Which implies that there just isn’t enough planet building material in the primordial dust disk, that far out.
As fas as I know, this means little or nothing with regard to terrestrial planets or ‘our type’ planetary system.

Abstract: Smaller terrestrial planets (less than 0.3 Earth masses) are less likely to retain the substantial atmospheres and ongoing tectonic activity probably required to support life. A key element in determining if sufficiently massive “sustainably habitable” planets can form is the availability of solid planet-forming material. We use dynamical simulations of terrestrial planet formation from planetary embryos and simple scaling arguments to explore the implications of correlations between terrestrial planet mass, disk mass, and the mass of the parent star. We assume that the protoplanetary disk mass scales with stellar mass as Mdisk ~ f Mstar [up arrow] h, where f measures the relative disk mass, and 1/2 less than h less than 2, so that disk mass decreases with decreasing stellar mass. We consider systems without Jovian planets, based on current models and observations for M stars. We assume the mass of a planet formed in some annulus of a disk with given parameters is proportional to the disk mass in that annulus, and show with a suite of simulations of late-stage accretion that the adopted prescription is surprisingly accurate.

Our results suggest that the fraction of systems with sufficient disk mass to form greater than 0.3 Earth mass habitable planets decreases for low-mass stars for every realistic combination of parameters. This “habitable fraction” is small for stellar masses below a mass in the interval 0.5 to 0.8 Solar masses, depending on disk parameters, an interval that excludes most M stars. Radial mixing and therefore water delivery are inefficient in lower-mass disks commonly found around low-mass stars, such that terrestrial planets in the habitable zones of most low-mass stars are likely to be small and dry.

“The odds are extremely slight that planets larger than four to five Jupiter masses exist at distances greater than 20 AU from these stars,” concluded Beth Biller of the UA Steward Observatory.

Hey – why should there be planets four-to-five times more massive than Jupiter in place where our Solar System spawned just mere Uranus? There’s simply not enough mass in protoplanetary nebula at that great distances to form super-giant planets, either by core accretion or gravitational instability models…

I would’ve thought that the obvious reaction to this data should be “Well that confirms what we always thought – no big planets a long way from their stars.”

This is the first finding that one of our pre-1995 prejudices about exoplanets is actually correct – that they don’t form a long way from their stars. Brown dwarfs can and do, as the data has confirmed.

This is a good finding, that maybe we’re on the right track to understanding exoplanet formation.

Indeed a good complementary method. It’s also most sensitive to systems where we look on the pole of the star. Radial velocity methods won’t work there, I suppose.

This result does not rule out large planets at large distances if we look along the plane of the system. It takes a while for an outer planet to reach the largest optical separation. But still good enough for statistics.

Abstract: Context: Disc fragmentation has been proposed as a possible mechanism for the formation of giant planets at close distances to solar-type stars. However, it is debatable whether this mechanism can function in the inner region of real discs.

Aims: To investigate the thermodynamics of discs and the probability of fragmentation. Methods: We use a newly developed method to treat the energy equation and equation of state, which accounts for radiative transfer effects in SPH simulations of protostellar discs. The different chemical and internal states of hydrogen and the properties of dust at different densities and temperatures (ice coated dust grains at low temperatures, ice melting, dust sublimation) are all taken into account by the new method.

Results: We present radiative hydrodynamic simulations of discs where the effects of the equation of state and energy equation are taken into account. We focus on the inner parts of discs, R

Abstract: We present a numerical study of rapid, so called type III migration for Jupitersized planets embedded in a protoplanetary disc. We limit ourselves to the case of inward migration, and study in detail its evolution and physics, concentrating on the structure of the corotation and circumplanetary regions, and processes for stopping migration. We also consider the dependence of the migration behaviour on several key parameters. We perform this study using the results of global, two-dimensional hydrodynamical simulations with adaptive mesh refinement. The initial conditions are chosen to satisfy the condition for rapid inward migration.

We find that type III migration can be divided into two regimes, fast and slow. The structure of the coorbital region, mass accumulation rate, and migration behaviour differ between these two regimes. All our simulations show a transition from the fast to the slow regime, ending type III migration well before reaching the star. The stopping radius is found to be larger for more massive planets and less massive discs. A sharp density drop is also found to be an efficient stopping mechanism. In the fast migration limit the migration rate and induced eccentricity are lower for less massive discs, but almost do not depend on planet mass. Eccentricity is damped on the migration time scale.

Abstract: We examine the implications for the distribution of extrasolar planets based on the null results from two of the largest direct imaging surveys published to date. Combining the measured contrast curves from 22 of the stars observed with the VLT NACO adaptive optics system by Masciadri et al. (2005), and 48 of the stars observed with the VLT NACO SDI and MMT SDI devices by Biller et al. (2007) (for a total of 60 unique stars; the median star for our survey is a 30 Myr K2 star at 25 pc), we consider what distributions of planet masses and semi-major axes can be ruled out by these data, based on Monte Carlo simulations of planet populations. We can set this upper limit with 95% confidence: the fraction of stars with planets with semi-major axis from 20 to 100 AU, and mass greater than 4 M_Jup, is 20% or less. Also, with a distribution of planet mass of dN/dM ~ M^-1.16 between 0.5-13 M_Jup, we can rule out a power-law distribution for semi-major axis (dN/da ~ a^alpha) with index 0 and upper cut-off of 18 AU, and index -0.5 with an upper cut-off of 48 AU. For the distribution suggested by Cumming et al. (2007), a power-law of index -0.61, we can place an upper limit of 75 AU on the semi-major axis distribution. At the 68% confidence level, these upper limits state that fewer than 8% of stars have a planet of mass greater than 4 M_Jup between 20 and 100 AU, and a power-law distribution for semi-major axis with index 0, -0.5, and -0.61 cannot have giant planets beyond 12, 23, and 29 AU, respectively. In general, we find that even null results from direct imaging surveys are very powerful in constraining the distributions of giant planets (0.5-13 M_Jup) at large separations, but more work needs to be done to close the gap between planets that can be detected by direct imaging, and those to which the radial velocity method is sensitive.

Abstract: We use numerical simulations to model the migration of massive planets at small radii and compare the results with the known properties of ‘hot Jupiters’ (extrasolar planets with semi-major axes a less than 0.1 AU). For planet masses Mp sin i greater than 0.5 MJup, the evidence for any `pile-up’ at small radii is weak (statistically insignificant), and although the mass function of hot Jupiters is deficient in high mass planets as compared to a reference sample located further out, the small sample size precludes definitive conclusions. We suggest that these properties are consistent with disc migration followed by entry into a magnetospheric cavity close to the star. Entry into the cavity results in a slowing of migration, accompanied by a growth in orbital eccentricity. For planet masses in excess of 1 Jupiter mass we find eccentricity growth timescales of a few x 10^5 years, suggesting that these planets may often be rapidly destroyed. Eccentricity growth appears to be faster for more massive planets which may explain changes in the planetary mass function at small radii and may also predict a pile-up of lower mass planets, the sample of which is still incomplete.

Formation of Hot Planets by a combination of planet scattering, tidal circularization, and Kozai mechanism

Authors: M. Nagasawa, S. Ida, T. Bessho

(Submitted on 9 Jan 2008)

Abstract: We have investigated the formation of close-in extrasolar giant planets through a coupling effect of mutual scattering, Kozai mechanism, and tidal circularization, by orbital integrations. We have carried out orbital integrations of three planets with Jupiter-mass, directly including the effect of tidal circularization. We have found that in about 30% runs close-in planets are formed, which is much higher than suggested by previous studies. We have found that Kozai mechanism by outer planets is responsible for the formation of close-in planets. During the three-planet orbital crossing, the Kozai excitation is repeated and the eccentricity is often increased secularly to values close enough to unity for tidal circularization to transform the inner planet to a close-in planet. Since a moderate eccentricity can remain for the close-in planet, this mechanism may account for the observed close-in planets with moderate eccentricities and without nearby secondary planets. Since these planets also remain a broad range of orbital inclinations (even retrograde ones), the contribution of this process would be clarified by more observations of Rossiter-McLaughlin effects for transiting planets.

Abstract: Gravitational fragmentation has been proposed as a mechanism for the formation of giant planets in close orbits around solar-type stars.
However, it is debatable whether this mechanism can function in the inner regions (R less than 40 AU) of real discs.

We use a newly developed method for treating the energy equation and the equation of state, which accounts for radiative transfer effects in SPH simulations of circumstellar discs. The different chemical and internal states of hydrogen and the properties of dust at different densities and temperatures (ice coated dust grains at low temperatures, ice melting, dust sublimation) are all taken into account by the new method.

We present radiative hydrodynamic simulations of the inner regions of massive circumstellar discs and examine two cases: (i) a disc irradiated by a cool background radiation field (T_bgr=10K)and (ii) a disc heated by radiation from its central star (T_bgr~1/R). In neither case does the disc fragment: in the former because it cannot cool fast enough and in the latter because it is not gravitationally unstable.

Our results (a) corroborate previous numerical results using different treatments for the hydrodynamics and the radiative transfer, and (b) confirm our own earlier analytic predictions. We conclude that disc fragmentation is unlikely to be able to produce giant planets around solar-type stars at radii less than 40 AU.

Comments: Accepted by A&A, 10 pages, high-resolution available at this http URL

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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